The present invention relates to polymers having very low birefringence and other properties making such polymers ideally suited for use in the substrates of optical storage disks and in other applications where good optical properties are desired. Optical storage disks such as compact disks, and more particularly, data storage disks having the ability to both record and playback digitalized data are currently of high commercial interest. Current designs generally employ a disk shaped substrate having good optical transmission properties and one or more layers deposited on at least one major surface of such disk for the inscription of digitalized information. The relatively simplistic compact disk is an example of a read-only optical storage disk. Generally such disks comprise only a reflective layer and a substrate wherein the desired information is encoded in a series of pits formed into the substrate by use of any suitable technique. More advanced designs adapted for both recording and playback of such data and even erasure and rerecording of such data may interpose one or more additional layers between the substrate and reflective layer, one of which is referred to as an active layer. Active layers are comprised of materials that permit data recording. Suitable materials include temperature sensitive photoactive materials, amorphous crystals, magnetically activated optical materials (magneto-optic materials) or other suitable compositions. In particular regard to magneto-optic active layers, a plane polarized laser beam utilized to abstract data from the optical disk is caused to be rotated by passing through the active layer. An optical sensing device such as a light sensitive diode is positioned to intercept the light beam and determine whether rotation has occurred. Rotation of the light beam by the active layer results in a null output signifying a zero. If the laser beam is unaffected by the active layer, the diode is adapted to sense the light thereby signifying a one. As the laser beam scans the surface of the disk a series of zeros and ones is recorded corresponding to the data inscribed in the active layer.
In the preparation of optical disks employing the above described magneto-optic active layer, a thermoplastic substrate is desirably employed. Such substrates may suitably be prepared by injection molding. A number of physical constraints must be satisfied by the polymeric material employed in the preparation of such substrates. Of course, the substrate must be optically transparent at the wavelength employed by the laser sensing means. In addition to being optically transparent, such substrate must not rotate the plane polarized laser beam in a manner that would result in obscuration of the desired output signal.
In other optical uses a low birefringent molding polymer is equally desirable. In such applications as glazing, or in the preparation of automobile interior instrument panel covers, the use of birefringent polymers may be undesirable due to the colored diffraction patterns that may appear under certain lighting conditions.
It is known that polymer anisotropy resulting in birefringence or light interference may result from stress that occurs during the molding of thermoplastic objects. To a certain extent this birefringence may be reduced by the use of molding techniques designed to achieve articles having little or no molded-in stress. These techniques include the use of low molecular weight polymers, high molding temperatures and increased molding times to promote polymer relaxation and the use of molds having large, unimpeded gates and channels to reduce the birefringence of the resulting molded object. Disadvantageously, such techniques increase the time required for molding disk substrates and ultimately increase molding costs.
It is previously known in the art that a material having low birefringence may be prepared by blending appropriate amounts of polymers having opposite birefringent properties. A blend comprising 82 percent polymethyl methacrylate and 18 percent polyvinylidene fluoride is disclosed in Polymer 26, 1619 (1985). Other blends having similar properties are disclosed in J. Hennig, Kunststoffe, 75, 425 (1985), U.S. Pat. No. 4,373,065, and CA 106 (2):11130A.
Disadvantageously, techniques utilizing polymer blending to achieve low birefringent materials require that the various components be completely miscible and that intimate blending techniques such as solution blending be employed to insure that no domains greater than the wavelength of light result. Large domain size could produce differing levels of birefringence in separate regions of the polymer. Furthermore, it has been discovered that a temperature/strain rate dependence can exist for such blends due to the fact that the two polymers may have independent relaxation rates. That is, if an object formed from such a blend is heated to a temperature above the glass transition temperature of less than all of the components of the blend, stress induced birefringence may reappear.
In addition to inherently low birefringence, a suitable polymer material for use in optical applications should also possess low thermal conductivity, low moisture absorption, excellent durability, and resistance to the effects of high heat. In particular, a desirable substrate material should exhibit a glass transition temperature high enough that techniques for deposition of a reflective layer such as metal sputtering or metal vapor disposition or other exposures to high temperatures do not affect the material. Also, for optical recording disks, low moisture absorption is desired such that differential expansion of the disk not occur and that interaction of water vapor with other components of the disk, especially either the active layer or the metallic reflective layer not take place. The durability or dimensional stability of the disk depends largely on the modulus of the polymer substrate. Finally, suitable resins for use as a substrate material should have sufficient tensile strength to be usefully employed as a molding resin in compression or injection molding applications.
In U.S. Pat. No. 4,785,053 an optical material having reduced birefringence comprising at least two constituent units each having positive and negative main polarizability differences of at least 50.times.10.sup.-25 cm.sup.3 in terms of absolute values and good water absorbance, heat resistance and moldability is disclosed. At col. 5, lines 45-47, the reference taught that copolymers of styrene and methylmethacrylate possess reduced birefringence. It has now been discovered that this teaching of the reference is incorrect due to reliance by the inventors on previously published values of polarizability for methylmethacrylate containing polymers (which were based on solution measurements of polymethylmethacrylate polarizability). Such solution measurements are not reliable for use in the prediction of ultimate polarizability of copolymeric species. Instead, measurements of molten polymer main polarizability must be employed. For example, the present inventors have now discovered that polymethylmethacrylate possesses a negative, not a positive polarizability value, when measured in a polymer melt. Since polystyrene also has a negative polarizability value, no ratio of styrene and methylmethacrylate monomers will result in reduction of the birefringence of the copolymer compared to polymethylmethacrylate. Contrary to the prediction of U.S. Pat. No. 4,785,053, such polymers comprising styrene and methacrylate esters do not possess reduced birefringence in the absence of a comonomer possessing positive homopolymer polarizability.
The reference also disclosed certain interpolymers of 45 parts by weight methylmethacrylate, 22 parts by weight styrene and 33 parts by weight n-butyl maleimide. Such polymers do hold a potential for reduced birefringence due to the presence of the n-butyl maleimide moiety which has a positive stress optic coefficient. Despite this fact, such copolymers are still unacceptable for use as high performance optical disk substrates. One defect in such copolymers is a still unacceptably high water absorption. Another defect is low strength properties of the resin.
Thus there remains a need to provide copolymers having zero or reduced birefringence in a predictable manner.
There also remains a need to provide a new polymer for use where a combination of good optical properties and strength properties are desired.
More particularly, it would be desirable to provide a polymer able to provide molded objects having reduced birefringence, easy moldability, low water absorption and high tensile strength and modulus properties.
Finally, it would be desirable to provide optical devices such as a substrates used in the preparation of optical disks, lenses, instrument cases and covers, etc., comprising the above copolymer.